1. Field of the Invention
The present invention relates to a pH sensor, and in particular to a planar solid-state reference electrode of a pH sensor.
2. Description of the Related Art
Since there are many drawbacks to the practical application of the conventional organic quantitative analysis, such as complicated operation, long analysis time, expensive equipments, inapplicability for the detection of a continuous process and the like. [J. U. Chen, Biological Industry, Vol. 4(3), 1993, pp. 205-212; D. G. Huang, W. S. Chen, and R. C. Hsu, Biological Industry, Vol. 7(4), 1996, pp. 291-298; S. Zhang, G. Wright, and Y. Yang, Biosensors and Bioelectronics, Vol. 15, 2000, pp. 273-282] Studies to discover a solution that can overcome disadvantages associated with the conventional quantitative analysis have been carried out. As a result, a biosensor is designed by combining the theories of biochemistry, electrical circuit, material science, and optics to meet the requirements of various fields.
The ion-sensitive field effect transistor (ISFET) was first disclosed by P. Bergveld in 1970 [P. Bergveld, IEEE Tran. on Biomed. Eng., Vol. BME-17, 1970, pp. 710-714]. The device is a product of applied electrochemistry and microelectronics, and has an ion selection function and FET properties. The ion-sensitive device is strictly different from the conventional ion selection electrode. The ISFET is a semiconductor pH sensor in which the metal on the gate of the metal-oxide semiconductor field effect transistor (MOSFET) is removed to expose the silicon dioxide layer. When the ISFET is placed into an aqueous solution, the exposed silicon dioxide layer detects the Zeta potential produced from the aqueous solution such that the purpose of sensing the ion concentration in the aqueous solution can be achieved. The related studies on ISFET, such as material improvement [Manuela Adami et al., Sensors and Actuators B, Vol. 24-25, 1995, pp. 889-893; A. S. Poghossian et al., Sensors and Actuators B, Vol. 7, 1992, pp. 367-370; T. Katsube et al., Sensors and Actuators B, Vol. 2, 1982, pp. 399-410], the study and miniaturization of reference electrodes [S. D. Collins et al., Sensors and Actuators B, Vol. 10, 1993, pp. 169-178; Yuri G. Vlasov, Andrey V. Bratov, Sensors and Actuators B, Vol. 10, 1992, pp. 1-6; C. Diekmann et al., Sensors and Actuators B, Vol. 24-25, 1995, pp. 276-278], the improvement of structures [C. Cane et al., Sensors and Actuators B, Vol. 35-36, 1996, pp. 136-140; Pavel Neuzil et al., Sensors and Actuators B, Vol. 24-25, 1995, pp. 232-235] and the like have been discussed.
An extended gate field effect transistor (EGFET) is an element developed from ISFET, provided firstly by J. Spiegel [J. Van Der Spiegel et al., Sensors and Actuators, Vol. 4, 1983, pp. 291-298], and unlike ISFET, the EGFET preserves the original gate in the MOSFET and has a sensing membrane plated on the other end extended from the metal gate. Compared with ISFET, the EGFET has the following advantages: (1) electrostatic protection is provided by the conductive wire on the element; (2) elimination of direct contact between the transistor of the element and the aqueous solution; (3) the effect of light on the element being reduced [P. Bergveld and A. Sibbald, “Analytical and Biomedical Application of Ion-Sensitive Field Effect Transistor”, published by Elsevier science, New York, 1988, pp. 2-60].
A reference electrode is a type of electrochemical sensing device, which is an electrode used to establish a standard reference potential corresponding to the different standard potential of the solution to be detected. The feature of the reference electrode is that the surface potential of the reference electrode remains stable in different solutions and avoids deviation of the sensitivity of the sensing device caused by different detected solutions. A reference electrode commonly used on an ordinary electrochemical sensing device is a calomel electrode or a silver/silver chloride electrode, but most reference electrodes are wet reference electrodes, and therefore, those reference electrodes cannot be miniaturized, and must be immersed into an associated buffer solution for a long period, which is inconvenient both for its use and storage. Hence, in order to achieve the objects of miniaturized fabrication and dry storage, the reference electrode design is an important subject of study and there are many related articles having discussions regarding this aspect. Referring to articles on pH ISFET, it is found that the miniaturization of a reference electrode is a present tendency of the sensing device development, while current methods of fabrication include: micro-electromechanical processing, silver/silver chloride membrane deposition, differential pair circuit design [Huixian Zhu et al., Sensors and Actuators B, Vol. 46, 1998, pp. 155-159; Joseph J. Pancrazio et al., Biosensors and Bioelectronics, Vol. 13, 1998, pp. 971-979; N. Zine et al., “Multisensor Silicon Needle for Cardiac Applications”, Proceedings of The 1st Annual International Conference on Microtechnologies in Medicine and Biology, 2000, pp. 216-219 R. J. Reay et al., “An Integrated CMOS Potentiostat for Miniaturized Electroanalytical Instrumentation”, Proceedings of the IEEE International Solid-State Circuits Conference, 1994, pp. 162-163].
Patents disclosing conventional techniques include: U.S. Pat. No. 6,251,246 to Andy D. C. Chan discloses a material for establishing solid-state contact for ion selective electrodes which is a polymeric material forming a stable, reproducible interface between the ionic and electronic domains of an ion selective sensor, or an ion selective field effect transistor, or the like. When employed in an ion selective sensor, the polymeric material provides a solid internal reference electrode and an ion selective material. U.S. Pat. No. 6,218,208 to the inventors discloses fabrication of a multi-structure ion sensitive field effect transistor with a pH sensing layer of a tin oxide thin film. The multi-structure ISFET has high performances such as a linear pH sensitivity of approximately about 56-58 mV/pH in a concentration range between pH 2 and pH 10, a low drift value of approximately 5 mV/day, and response time of less than 0.1 seconds. This device has other advantages, such as the inexpensive fabrication system, low cost, and mass production.
U.S. Pat. No. 5,309,085 to Byung Ki Sohn discloses a measuring circuit with a biosensor utilizing ion sensitive field effect transistors. The circuit has advantages of being a simple structure and easy to integrate, which comprises two ISFETs as inputs, one is an enzyme field effect transistor (enzyme EFT), and the other is the reference FET. The circuit has various amplification functions to amplify the sensed output of the sensing device. The voltage variation of ISFET was raised through using an unsteady semi-reference electrode that could be affected by the change of the temperature, so that the working characteristic of the device could be adjusted by changing the gain of read-out circuit. The ISFET biosensor can be provided on a single chip in combination with a measuring circuit to achieve the miniaturization of the sensing device. U.S. Pat. No. 5,296,122 to Teruaki Katsube, Shuichiro Yamaguchi, Naoto Uchida, and Takeshi discloses an apparatus for forming thin film which is a hydrophobic membrane to be used as the reference electrode of an ISFET. The hydrophobic membrane is grown on a substrate by neutral plasma deposition or by sputtering. The apparatus includes a vacuum chamber, an atom beam generator, a target base, and a shield for growth controlling. The thin membrane was suitable for an ion sensor, such as an ISFET, and an enzyme sensor.
U.S. Pat. No. 4,641,084 to Satsuki Komatsu discloses an apparatus for measuring ion concentration of a specific ion contained in a test liquid. The measurement is performed with the aid of a reference electrode and an ion sensitive field effect transistor having a gate portion selectively sensitive to the specific ion, including a series circuit of a reference resistor and a constant voltage supply source connected across drain and source of the ion sensitive field effect transistor, a potential control circuit having inputs connected across the reference resistor to detect a potential difference across the reference resistor for controlling a source or drain potential of the ion sensitive field effect transistor, in such a manner that the potential difference remains at a predetermined value, and a voltmeter for measuring the source or drain potential as a measure of the ion concentration. In addition, U.S. Pat. No. 5,602,467 to Mathias Krauss, Beate Hildebrandt, Christian Kunath, and Eberhard Kurth discloses a circuit for measuring ion concentrations in solutions. A framework for measuring the ion concentration in the solution by using an ISFET circuit layout is provided. The circuit layout could expose the gate voltage difference of the FET and the parameter/environmental deviation caused by operation factors. The circuit layout comprises two measurement/test amplifiers, two ISFETs, and two identical FETs. The two ISFETs are connected to the two FET respectively, and the output from the first amplifier displays the gate voltage change between two ISFETs and FETs, and the second amplifier displays the output difference of two ISFET. The output of the first amplifier is the ground reference electrode that connected to 4 reference electrodes. Thus, the framework is capable of detecting the ion concentration.
U.S. Pat. No. 4,882,292 to Ernst J., and Maria D. discloses a process for manufacturing a REFET or a CHEMFET. The process for manufacturing a REFET and/or CHEMFET comprises (a) covalent bonding of a hydrophilic polymer layer to an isolator layer applied to a semiconductor material; (b) the absorption of water or an aqueous solution into said hydrophilic polymer layer; and (c) the binding of a hydrophobic polymer layer to the water holding hydrophilic polymer layer.
U.S. Pat. No. 5,684,619 to Shabrang Mani, Babinec Susan J., and Varjian Richard D. discloses an improved electrochromic device. The improved electrochromic device has an electrochromic electrode in contact with an ion conductor, and the ion conductor in turn being in contact with a layer of gold. The improvement is to interpose a layer of ruthenium oxide between the ion conductor and the layer of gold. The layer of ruthenium oxide is not an electrochromic material.
From these disclosures, it can be seen that the conventional techniques still have many drawbacks and are not designed well, and the improvement for a solid-state dry reference electrode and a planar sensing device framework is still required.